类常量

可以把在类中始终保持不变的值定义为常量。在定义和使用常量的时候不需要使用 $ 符号。

常量的值必须是一个定值，不能是变量，类属性，数学运算的结果或函数调用。

接口（interface）中也可以定义常量。更多示例见文档中的接口部分。

自 PHP 5.3.0 起，可以用一个变量来动态调用类。但该变量的值不能为关键字（如 self，parent 或 static）。

Example #1 定义和使用一个类常量

<?php
class MyClass
{
    const constant = 'constant value';

    function showConstant() {
        echo  self::constant . "\n";
    }
}

echo MyClass::constant . "\n";

$classname = "MyClass";
echo $classname::constant . "\n"; // 自 5.3.0 起

$class = new MyClass();
$class->showConstant();

echo $class::constant."\n"; // 自 PHP 5.3.0 起
?>

Example #2 静态数据示例

<?php
class foo {
    // 自 PHP 5.3.0 起
    const bar = <<<'EOT'
bar
EOT;
}
?>

和 heredoc 不同，nowdoc 可以用在任何静态数据中。

Note:

Nowdoc 支持是在 PHP 5.3.0 新增的。

User Contributed Notes

ardv five two six at gmail dot com 01-Sep-2018 06:06
I think note "tmp dot 4 dot longoria at gmail dot com" may be some extend in this note.

<?php
abstract class dbObject
{   
    const TABLE_NAME='parentConst';
   
    public static function GetAll()
    {
        $c = get_called_class();
        return "SELECT * FROM `".$c::TABLE_NAME."`";
    }      
    public static function getChildConst()
    {
        return "SELECT * FROM `".static::TABLE_NAME."`"; // Late Static Bindings
    }   
    public static function getParentConst()
    {
        return "SELECT * FROM `".self::TABLE_NAME."`";
    }
}

class dbPerson extends dbObject
{    
    const TABLE_NAME='childPersonsConst';
}

class dbAdmin extends dbPerson
{
    const TABLE_NAME='childAdminsConst';
}

echo dbPerson::GetAll()."<br>";//output: "SELECT * FROM `childPersonsConst`"
echo dbAdmin::GetAll()."<br>";//output: "SELECT * FROM `childAdminsConst`"

echo dbPerson::getChildConst()."<br>";//output: "SELECT * FROM `childPersonsConst`"
echo dbAdmin::getChildConst()."<br>";//output: "SELECT * FROM `childAdminsConst`"

echo dbPerson::getParentConst()."<br>";//output: "SELECT * FROM `parentConst`"
echo dbAdmin::getParentConst()."<br>";//output: "SELECT * FROM `parentConst`"
?>
David Spector 16-Aug-2018 01:55
The usual comma-separated syntax can be used to declare several constants:

class STATE
    {
    const INIT=0, NAME_SEEN=1, ADDR_SEEN=2;
    }

This shows the declaration of a set of enumeration literals suitable for use in a finite state machine loop. Reference such an enum by using syntax such as "STATE::INIT". Its actual type in this case will be integer.
dubious 17-Feb-2018 11:29
If absolutely necessary you can set class constants to variables like this:

$temp_wfr = substr(dirname(__FILE__),0,-3); // temp site root directory
$toeval = <<<TOEVAL
class Server extends Server_Base {
    const LOCATION = "development";
    const WEBFILEROOT = "$temp_wfr";
}
TOEVAL;
eval($toeval);

While I wouldn't use this in production, it came in very handy in my development environment where my directory locations were changing with each test version of the site being developed.  Server::WEBFILEROOT was used throughout the site, so setting it automatically saved me a lot of time.
Nimja 06-Sep-2017 01:07
Note that this magic constant DOES NOT load classes. And in fact can work on classes that do not exist.

This means it does not mess with auto-loading.

<?php
    $className = \Foo\Bar::class;
    var_dump($className);
    var_dump(class_exists($className, false));
?>

Will output:

    string(7) "Foo\Bar"
    bool(false)
scohen987 at gmail dot com 18-Aug-2017 06:16
All non-class constants, declared or not, also have a magic class constant, ie:

    >>> FOO
    PHP error:  Use of undefined constant FOO - assumed 'FOO' on line 1
    >>> FOO::class
    => "FOO"
    >>> define('FOO','BAR')
    => true
    >>> FOO
    => "BAR"
    >>> FOO::class
    => "FOO"
Anonymous 17-Jul-2017 07:40
Before version 7.1.0 you couldn't write something like this:
<?php
class foo{

private const myvar = 13;
...
}
?>

and an if  with version_compare isn't also allowed how could i write the code for it so the class is compatible from 5.6 - 7.1.0?

Also I don't understand which are the advantages of modifying the class scope of a constant because constant couldn't be change at runtime.

Maybe someone could clarify this
jimmmy dot chief at gmail dot com 25-Oct-2016 02:57
Hi, i would like to point out difference between self::CONST and $this::CONST with extended class.
Let us have class a:

<?php
class a {   
    const CONST_INT = 10;
   
    public function getSelf(){
        return self::CONST_INT;
    }
   
    public function getThis(){
        return $this::CONST_INT;
    }
}
?>

And class b (which extends a)

<?php
class b extends a {
    const CONST_INT = 20;
   
    public function getSelf(){
        return parent::getSelf();
    }
   
    public function getThis(){
        return parent::getThis();
    }
}
?>

Both classes have same named constant CONST_INT.
When child call method in parent class, there is different output between self and $this usage.

<?php
$b = new b();

print_r($b->getSelf());     //10
print_r($b->getThis());     //20

?>
nepomuk at nepda dot de 15-Mar-2016 08:50
[Editor's note: that is already possible as of PHP 5.6.0.]

Note, as of PHP7 it is possible to define class constants with an array.

<?php
class MyClass
{
    const ABC = array('A', 'B', 'C');
    const A = '1';
    const B = '2';
    const C = '3';
    const NUMBERS = array(
        self::A,
        self::B,
        self::C,
    );
}
var_dump(MyClass::ABC);
var_dump(MyClass::NUMBERS);

// Result:
/*
array(3) {
    [0]=>
  string(1) "A"
    [1]=>
  string(1) "B"
    [2]=>
  string(1) "C"
}
array(3) {
    [0]=>
  string(1) "1"
    [1]=>
  string(1) "2"
    [2]=>
  string(1) "3"
}
*/
?>
info at stanzentech dot com 09-Jun-2015 08:43
<?php
//http://php.net/manual/en/language.oop5.constants.php
/**
 * Constant name shouldn't start with $
 * Constant name may lower or uppercases.
 * Same constant name can be used as a property name but must start with $ symbol.
 * Constant doesn't available with $this-> inside class definition.
 * Constant is available with self:: inside class definition.
 * Constant can't call with $this-> outside class.
 * Constant is accessible with :: after "Class Name or Object".
 *
 */
class MyClass
{       
    // Parse error: syntax error, unexpected '$CONSTANT' (T_VARIABLE), expecting identifier (T_STRING) in constant.php
    //const $CONSTANT      =   'constant named "CONSTANT" ';
 
    const CONSTANT      =   'constant named "CONSTANT" ';
    const small         =   'constant named "small" ';
   
    public $small        =   'SAME CONTSNAT NAME AS PROPERTIES.'; 

    //Fatal error: Cannot redefine class constant MyClass::small in constant.php
    // const small         =   'constant named "small" ';
       
    function showConstant() {
        echo  self::CONSTANT . "<br>";
        //echo $this->CONSTANT . "<br>"; // Notice: Undefined property: MyClass::$CONSTANT in constant.php
    }
}

$class      =   new MyClass();
$class->showConstant();

//Notice: Undefined property: MyClass::$CONSTANT in constant.php
//echo $class->CONSTANT."<br>";

echo $class->small."<br>";  // SAME CONTSNAT NAME AS PROPERTIES.
?>
Paul 12-May-2015 10:44
Square or curly bracket syntax can normally be used to access a single byte (character) within a string. For example: $mystring[5]. However, please note that (for some reason) this syntax is not accepted for string class constants (at least, not in PHP 5.5.12).
For example, the following code gives "PHP Parse error:  syntax error, unexpected '[' in php shell code on line 6".
<?php
class SomeClass
{
  const SOME_STRING = '0123456790';
  public static function ATest()
  {
    return self::SOME_STRING[0];
  }
}
?>
It looks like you have to use a variable/class member instead.
kuzawinski dot marcin at gmail dot com 11-Nov-2014 01:27
As of PHP 5.6 you can finally define constant using math expressions, like this one:

<?php

class MyTimer {
    const SEC_PER_DAY = 60 * 60 * 24;
}

?>

Me happy :)
enrico_kaelert at kabelmail dot com 01-May-2014 07:05
additional to tmp dot 4 dot longoria at gmail dot com ′s post:
quote:
it's possible to declare constant in base class, and override it in child,
/quote

Its not that we overwrite them.
Its more that each got its own:
<?php
abstract class dbObject
{
    const TABLE_NAME='undefined';
}

class dbPerson extends dbObject
{
    const TABLE_NAME='persons';

    public static function getSelf()
    {
        return self::TABLE_NAME;
    }
    public static function getParent()
    {
        return parent::TABLE_NAME;
    }
}

class dbAdmin extends dbPerson
{
    const TABLE_NAME='admins';

    public static function getSelf()
    {
        return self::TABLE_NAME;
    }
    public static function getParent()
    {
        return parent::TABLE_NAME;
    }
}

echo '<pre>
im class dbPerson{} and this is my:
    self TABLE_NAME:    '.dbPerson::getSelf().'   // persons
    parent TABLE_NAME: '.dbPerson::getParent().'  // undefined

im class dbAdmin{} and this is my:
    self TABLE_NAME:   '.dbAdmin::getSelf().'    // admins
    parent TABLE_NAME: '.dbAdmin::getParent().'  // persons
';
?>

or more readable:
<?php
class ParentClass
{
    const CONSTANT = 'CONST_PARENT';
}

class A extends ParentClass
{
    const CONSTANT = 'CONST_A';

    public static function getSelf()
    {
        return self::CONSTANT;
    }
    public static function getParent()
    {
        return parent::CONSTANT;
    }
}

echo '<pre>
im class A{} and this is my:
    self CONSTANT:    '.A::getSelf().'   // CONST_A
    parent CONSTANT: '.A::getParent().'  // CONST_PARENT
';
?>
Xiong Chiamiov 10-Mar-2014 11:36
const can also be used directly in namespaces, a feature never explicitly stated in the documentation.

<?php
# foo.php
namespace Foo;

const BAR = 1;
?>

<?php
# bar.php
require 'foo.php';

var_dump(Foo\BAR); // => int(1)
?>
delete dot this dot and dot dots dot gt at kani dot hu 16-Jan-2014 11:05
I think it's useful if we draw some attention to late static binding here:
<?php
class A {
    const MY_CONST = false;
    public function my_const_self() {
        return self::MY_CONST;
    }
    public function my_const_static() {
        return static::MY_CONST;
    }
}

class B extends A {
   const MY_CONST = true;
}

$b = new B();
echo $b->my_const_self ? 'yes' : 'no'; // output: no
echo $b->my_const_static ? 'yes' : 'no'; // output: yes
?>
keenskelly at gmail dot com 13-Dec-2013 08:42
Re: "The value must be a constant expression, not (for example) a variable, a property, a result of a mathematical operation, or a function call."

I dare say that "a mathematical operation" can indeed be a constant expression. I was quite surprised by this limitation; you cannot, for example do something like:

const LIMITMB = 20;
const LIMITB = LIMITMB * 1024 * 1024;

It is very common to be able to express something like that in other languages, like C with #defines, where changing one definition has a cascading effect on others without having to pre-calculate hard-coded numbers all over the place. So beware, you might be better off using a private static or global contstant definition if you need to do anything more sophisticated than a name=primitive value pair.
Anonymous 29-Oct-2013 11:38
Noted by another is that class constants take up memory for every instance. I cannot see this functionality being accurate, so testing thusly:

class SomeClass {
 const thing = 0;
 const thing2 = 1;
}

$m0 = memory_get_usage();
$p0 = new SomeClass();
$p1 = new SomeClass();
$p2 = new SomeClass();
$m1 = memory_get_usage();
printf("memory %d<br />", $m1 - $m0);

The output does not change when one alters the count of constants in "SomeClass".
jaimz at vertigolabs dot org 28-Oct-2013 06:01
I thought it would be relevant to point out that with php 5.5, you can not use self::class, static::class, or parent::class to produce a FQN. Doing so produces a PHP Parse error:

"PHP Parse error:  syntax error, unexpected 'class' (T_CLASS), expecting variable (T_VARIABLE) or '$'"

It would be nice if you could do this however.
tmp dot 4 dot longoria at gmail dot com 04-Jun-2011 01:52
it's possible to declare constant in base class, and override it in child, and access to correct value of the const from the static method is possible by 'get_called_class' method:
<?php
abstract class dbObject
{   
    const TABLE_NAME='undefined';
   
    public static function GetAll()
    {
        $c = get_called_class();
        return "SELECT * FROM `".$c::TABLE_NAME."`";
    }   
}

class dbPerson extends dbObject
{
    const TABLE_NAME='persons';
}

class dbAdmin extends dbPerson
{
    const TABLE_NAME='admins';
}

echo dbPerson::GetAll()."<br>";//output: "SELECT * FROM `persons`"
echo dbAdmin::GetAll()."<br>";//output: "SELECT * FROM `admins`"

?>
jakub dot lopuszanski at nasza-klasa dot pl 14-Nov-2010 11:20
[Editor's note: that behavior has changed as of PHP 7.0.0, though.]

Suprisingly consts are lazy bound even though you use self instead of static:
<?php
class A{
  const X=1;
  const Y=self::X;
}
class B extends A{
  const X=1.0;
}
var_dump(B::Y); // float(1.0)
?>
anonymous 27-Sep-2010 06:32
Most people miss the point in declaring constants and confuse then things by trying to declare things like functions or arrays as constants. What happens next is to try things that are more complicated then necessary and sometimes lead to bad coding practices. Let me explain...

A constant is a name for a value (but it's NOT a variable), that usually will be replaced in the code while it gets COMPILED and NOT at runtime.

So returned values from functions can't be used, because they will return a value only at runtime.

Arrays can't be used, because they are data structures that exist at runtime.

One main purpose of declaring a constant is usually using a value in your code, that you can replace easily in one place without looking for all the occurences. Another is, to avoid mistakes.

Think about some examples written by some before me:

1. const MY_ARR = "return array(\"A\", \"B\", \"C\", \"D\");";
It was said, this would declare an array that can be used with eval. WRONG! This is just a string as constant, NOT an array. Does it make sense if it would be possible to declare an array as constant? Probably not. Instead declare the values of the array as constants and make an array variable.

2. const magic_quotes = (bool)get_magic_quotes_gpc();
This can't work, of course. And it doesn't make sense either. The function already returns the value, there is no purpose in declaring a constant for the same thing.

3. Someone spoke about "dynamic" assignments to constants. What? There are no dynamic assignments to constants, runtime assignments work _only_ with variables. Let's take the proposed example:

<?php
/**
 * Constants that deal only with the database
 */
class DbConstant extends aClassConstant {
    protected $host = 'localhost';
    protected $user = 'user';
    protected $password = 'pass';
    protected $database = 'db';
    protected $time;
    function __construct() {
        $this->time = time() + 1; // dynamic assignment
    }
}
?>

Those aren't constants, those are properties of the class. Something like "this->time = time()" would even totally defy the purpose of a constant. Constants are supposed to be just that, constant values, on every execution. They are not supposed to change every time a script runs or a class is instantiated.

Conclusion: Don't try to reinvent constants as variables. If constants don't work, just use variables. Then you don't need to reinvent methods to achieve things for what is already there.
wbcarts at juno dot com 12-Sep-2008 12:12
Use CONST to set UPPER and LOWER LIMITS

If you have code that accepts user input or you just need to make sure input is acceptable, you can use constants to set upper and lower limits. Note: a static function that enforces your limits is highly recommended... sniff the clamp() function below for a taste.

<?php

class Dimension
{
  const MIN = 0, MAX = 800;

  public $width, $height;

  public function __construct($w = 0, $h = 0){
    $this->width  = self::clamp($w);
    $this->height = self::clamp($h);
  }

  public function __toString(){
    return "Dimension [width=$this->width, height=$this->height]";
  }

  protected static function clamp($value){
    if($value < self::MIN) $value = self::MIN;
    if($value > self::MAX) $value = self::MAX;
    return $value;
  }
}

echo (new Dimension()) . '<br>';
echo (new Dimension(1500, 97)) . '<br>';
echo (new Dimension(14, -20)) . '<br>';
echo (new Dimension(240, 80)) . '<br>';

?>

- - - - - - - -
 Dimension [width=0, height=0] - default size
 Dimension [width=800, height=97] - width has been clamped to MAX
 Dimension [width=14, height=0] - height has been clamped to MIN
 Dimension [width=240, height=80] - width and height unchanged
- - - - - - - -

Setting upper and lower limits on your classes also help your objects make sense. For example, it is not possible for the width or height of a Dimension to be negative. It is up to you to keep phoney input from corrupting your objects, and to avoid potential errors and exceptions in other parts of your code.